1. Technical Field
The present invention relates to piezoelectric resonators, piezoelectric oscillators, electronic devices and methods for manufacturing piezoelectric resonators.
2. Related Art
A piezoelectric resonator that uses a piezoelectric vibrating element having thickness shear vibration as its main vibration, such as, an AT-cut quartz resonator is known (see Patent Document JP-A-2006-217253). Piezoelectric devices such as piezoelectric resonators are used in piezoelectric oscillators that are mounted, for example, on information communication devices such as cellular phones and electronic apparatuses such as computers. In this piezoelectric device, a beam-like piezoelectric vibrating element is supported within a piezoelectric device in a cantilever manner, using silicone-base conductive adhesive. The piezoelectric vibrating element is air-tightly sealed, for example, within a cavity that may be vacuum sealed during its manufacturing process. This is conducted for the purpose of protecting the piezoelectric device from external frequency disturbing factors such as dusts after shipping out from the piezoelectric device manufacturer, so as to maintain the resonance frequency at the time of shipping.
However, it has been reported that a frequency lowering phenomenon occurs with time in such a piezoelectric device after the device has been shipped out from the piezoelectric device manufacture and implemented into a product (see Patent Document JP-A-2006-217253). According to the Patent Document JP-A-2006-217253, the frequency lowering phenomenon occurring with time is caused by the fact that cyclic dimethylpolysiloxanes molecular components (hereafter called as “dimethylsiloxanes”) of silicone molecules evaporated from the silicone resin of the hardened silicone-base conductive adhesive are chemically adsorbed to the surface of the piezoelectric vibrating element within the cavity after shipping, thereby causing an increase in the mass of the piezoelectric vibrating element.
The silicone-base conductive adhesive that becomes a generation source of dimethylsiloxanes is very useful as it meets the demand for shock resistance, and it is therefore currently difficult to replace silicone-base conductive adhesive with other types of adhesive. Furthermore, in recent years, due to the demands for smaller and higher frequency piezoelectric devices, package bodies of piezoelectric resonators have been reduced in size, and the frequency band of resonance frequencies to be used has become higher, and therefore a possibility in that even a small increase in the mass of the piezoelectric vibrating element could lower the reliability of the resonance frequency of the piezoelectric vibrating element has been pointed out. Therefore, it is desired to provide piezoelectric resonators, whose resonance frequency is stable with time, and that are highly reliable.